What if old roof rights agreement misused to claim property?

In the year 1999, we received a draft in exchange for the roof rights of our dwelling house. We signed the agreement but did not use the draft and returned it to the person, from whom we received a money receipt in return. However, the original agreement paper is still with that person. Later, we handed over our dwelling house to a builder in September 2019. In 2020, that person filed a title suit before the Learned 2nd Civil Judge (Junior Division), Alipore, against my mother, brother, and me, along with the builder, claiming 50% of the ground floor with a 2 BHK flat, based on the roof rights mentioned in the old agreement paper.